Coupling biomass carbon engineering with phosphorus chemistry for high-performance flexible zinc-ion hybrid supercapacitors

Abstract

Efficient conversion of biomass into functional carbon materials is critical yet challenging for advanced energy storage systems (ESSs). We report a phosphoric acid activation strategy that yields N, O, and P co-doped porous carbon (FSP40-700) from fennel seeds. The material integrates hierarchical porosity, uniform heteroatom doping, and abundant defects, offering rich active sites, fast ion transport, and improved electrochemical kinetics in aqueous zinc ion hybrid supercapacitors (ZIHSCs). Phosphorus in the carbon structure helps in strengthening the existing donor sites and framework polarization, promoting directional anchoring of Zn²⁺ ions on the oxygenated edge sites with strong local electronic stabilization. These traits were validated through in situ operando Raman spectroscopy, in situ electrochemical impedance spectroscopy (EIS), density functional theory (DFT), and molecular dynamics (MD) simulation studies. As a ZIHSC cathode in aqueous electrolyte, FSP40-700 delivers a specific capacitance of 226 F g⁻¹ at 0.2 A g⁻¹ and retains 70% capacity after 40 000 cycles. Furthermore, a flexible ZIHSC device with hydrogel electrolyte achieves a high energy density of 123 Wh kg⁻¹ within a wide 0–2.0 V window, combining mechanical durability with excellent electrochemical performance, highlighting its promise for next-generation portable and wearable ESSs.